1. Introduction
Although primary and secondary preventive measures (especially statin and antiplatelet therapies) have reduced the risk of death due to cardiovascular diseases to a certain extent, cardiovascular diseases caused by atherosclerosis remain the main chronic diseases globally, with high mortality and disability rates, medical risks, and medical costs [1]. Thus, developing effective therapies remains a challenge in the management of millions of patients with atherosclerosis.
Growing evidence indicates that inflammation is a key regulator of atherosclerosis development and fragile plaque formation [2]. Cell death, especially necroptosis, is closely associated with inflammation. Lin et al. reported a large number of necroptotic cells and upregulated expression of receptor interacting protein kinase-3 (RIP3) in the necrotic core of progressive plaques [3]. Moreover, in human carotid plaques, the expression of necroptosis-related proteins such as RIP3 pseudokinase mixed lineage kinase domain-like (MLKL), and phosphorylated MLKL is upregulated [4]. Thus, cell necroptosis may be a promising target for the treatment of atherosclerosis [5]. In addition, using both non-lineage and vascular smooth muscle cell (VSMC) lineage-tracing models, recent studies have shown that similar to human coronary atheromas, VSMCs contribute to the majority of cells in ApoE−/− mouse atherosclerosis plaques [6,7] and undergo various forms of cell death, typically lead to the formation of a lipid-rich necrotic core within the evolving intimal lesion. Therefore, regulating the death of VSMC-derived foam cells will provide clinical benefits. Multiple pathogenic factors associated with cardiovascular diseases can cause cell necroptosis. Oxidized low-density lipoprotein (ox-LDL), a major proatherogenic factor, exerts a proinflammatory effect [8] and is rich in plaque necrotic core. However, whether ox-LDL induces the necroptosis of VSMC-derived foam cell and enlargement of the necrotic core is unknown.
Ox-LDL causes reactive oxygen species (ROS) to accumulate abnormally in cells, and ROS are key targets in necroptosis regulation [9]. Therefore, we hypothesized that ROS may play an important role in ox-LDL-induced cell necroptosis. In our previous study, we found that C1q/TNF-related protein 9 (CTRP9) inhibits cholesterol-induced inflammatory cytokine secretion and monocyte adhesion in VSMCs by activating the AMPK pathway [10]. Activated AMPK can inhibit oxidative stress-induced mitochondrial dysfunction [11] and is essential to understand whether CTRP9 plays a protective role against ox-LDL-induced necroptosis in VSMC-derived foam cells and atherosclerosis. In this study, we aimed to evaluate the effect of ox-LDL on VSMC-derived foam cell necroptosis, determine the role of VSMC-derived foam cell necroptosis in inflammation, and examine whether and how CTRP9 protects against ox-LDL-induced cell dysfunction. Furthermore, we determined the effect of CTRP9 on atherosclerosis. Our study could help in understanding the initiation and development of atherosclerosis and a serve as a potential therapeutic target for atherosclerosis.